Wavelength dependence of the mechanisms governing the formation of nanosecond laser-induced damage in fused silica.

The influence of the wavelength on the morphology of nanosecond laser-induced damage on the exit surface of fused silica is investigated. A combination between the typical features of damage sites initiated at 1064 nm and 355 nm is observed at 532 nm, including ring patterns sporadically exhibited, in good agreement with calculations of the development of an electron avalanche at this wavelength. The associated ring appearance speed scales as the cube root of the laser intensity, and is ~10.5 km/s while it is ~20 km/s when initiated by infrared pulses. The whole set of results sheds light on the different wavelength-dependent mechanisms governing damage formation.

Nanosecond laser damage initiation at 0.35 µm in fused silica.

In nanosecond regime, the laser-induced damage density at the exit surface of fused silica optics at the wavelength of 0.35 µm shows a characteristic behavior: in a specific fluence range, the surface damage density begins to grow exponentially as a function of fluence and then tends to saturate at high fluences. Up to now, no satisfactory explanation of these peculiarities could be provided. We herein detail a statistical model based on laser-matter interaction, where two types of absorbing precursors are involved in the energy deposit: subsurface micro-cracks and surface impurities. We show that the reported model predicts this characteristic damage density for a large range of fluences and different polishing processes.

Comparison of laser-induced surface damage density measurements with small and large beams: toward representativeness.

Pulsed laser damage density measurements obtained with diverse facilities are difficult to compare, due to the interplay of numerous parameters, such as beam area and pulse geometry, which, in operational large beam conditions, are very different from laboratory measurements. This discrepancy could have a significant impact; if so, one could not even pretend that laser damage density control is a real measurement process. In this paper, this concern is addressed. Tests with large beams of centimeter size on a high-power laser facility have beam performed according to a parametric study and are compared to small beam laboratory tests. It is shown that laser damage densities obtained with large and small beams are equal, within calculated error bars.